Editor’s note: This is a guest post by Sierra McMurry, a Ph.D. candidate in NC State’s College of Natural Resources.
Humans often start to slow down when autumn arrives, but bears are just getting started. Bears may seem like beasts without a schedule, but this is their busiest time of the year. They are up at all hours of the day consuming as much food as they can. The forest floor in autumn is covered in vibrant leaves, but there are also small, golden nuggets full of calories for bears: acorns.
Acorns represent far more than a simple snack or the beginnings of a new forest. They are the cornerstone of what ecologists call “mast,” the collective term for the nuts, seeds and fruits that trees produce in massive quantities during certain years. For North Carolina’s black bears, mast years can mean the difference between thriving and merely surviving the long winter months ahead.
The relationship between bears and mast is one of nature’s most compelling examples of ecological interdependence. When oak trees across the Appalachian Mountains synchronize their acorn production (an event that occurs roughly every two to three years), the forest transforms into a bear’s paradise. A single mature oak can drop thousands of acorns, and when multiple species of oak coordinate their timing, the result is a windfall that can sustain entire bear populations.
The bears’ autumn feeding behavior represents a remarkable physiological adaptation that scientists are only beginning to understand. Research in the Southern Appalachians revealed that during the “predenning hyperphagic” period, when bears are at their hungriest, black bears undergo systemic and hormonal changes that increase their ability to digest and metabolize fats and carbohydrates while decreasing protein assimilation. In other words, at this time of year bears pack on fat reserves for hibernation rather than build muscle.
By decreasing protein absorption, a bear’s digestive system is essentially saying “we don’t need muscle-building materials right now – we need fat storage.” This metabolic shift allows bears to efficiently convert the energy-rich acorns into body fat, the fuel they’ll need to survive the long winter months ahead. Preliminary digestibility trials on penned bears confirm that this isn’t simply about eating more food. It’s about the bear’s entire digestive system reorganizing itself to maximize energy storage from the specific nutrients that acorns provide.
I am part of a team of researchers at NC State University that is studying how these mast events impact mammal communities across North America. Researchers know that the timing of mast “drops” couldn’t be more crucial. Black bears in North Carolina need to gain 30 to 40 percent of their body weight before going into their dens to hibernate. During heavy mast years, we often see higher survival rates and more successful reproduction the following spring.
The bears’ foraging behavior during these autumn months is a marvel of efficiency. They can consume up to 20,000 calories per day (the equivalent of 35 hamburgers) with mast providing a perfect package of fats and carbohydrates. A single bear might eat thousands of acorns daily, their powerful jaws cracking through shells with mechanical precision. Bears have even developed techniques for maximizing their harvest, standing on hind legs to shake branches or climbing surprisingly high into oak canopies despite their considerable bulk.
What many hikers might mistake for storm damage in autumn forests is actually evidence of bears’ extraordinary feeding behavior. Black bears climb to the tops of oak trees exceeding 30 meters in height, using their strong, recurved claws to scale even smooth-barked trees with relative ease. Evidence of their aerial pursuits litters the forest floor: oak limbs snapped and scattered beneath feeding sites, elaborate “nests” created by bears pulling in branches to create feeding platforms, and historic scarring on tree trunks from repeated climbing seasons. During this “feeding frenzy,” bears abandon their typical crepuscular* activity patterns and move almost continuously, day and night, in pursuit of food. Some bears will even abandon readily available berry crops and cease their usual foraging behaviors to focus entirely on acorn-producing areas. [*Crepuscular means they are normally active at dawn and dusk.]
But mast production is notoriously unpredictable. Trees invest enormous energy in producing these crops, which means they often follow heavy mast years with periods of minimal production. This boom-and-bust cycle creates a ripple effect throughout the ecosystem. During poor mast years, bears are forced to travel greater distances, venture into human-populated areas more frequently, and compete more intensely for alternative food sources like berries, insects, and small mammals.
The quest for acorns drives dramatic shifts in bear movement patterns across the landscape. In the Southern Appalachians, bears respond to acorn abundance by moving to specific elevations, slopes and aspects containing the highest concentrations of oak trees. During poor mast years, bears travel two to four times farther than normal to reach suitable food sources, with some individuals completely abandoning their traditional spring-summer ranges for small pockets of concentrated acorn production. These movement patterns aren’t random. Bears systematically move from northwestern facing slopes in Tennessee to southeastern facing slopes in North Carolina, taking advantage of the higher concentration of white oaks on southeastern aspects.
The implications extend beyond individual bears. Female bears that enter winter well-nourished from abundant mast are more likely to successfully birth and nurse cubs during their winter denning period. Conversely, lean years can lead to increased cub mortality and delayed reproduction. Research has revealed the profound impact that mast production cycles have on bear populations.
Studies in New Mexico found that birth rate among females that have previously given birth decreased by more than 60% following years categorized as oak “failure,” and survival of young bears to breeding age decreased by more than 70% two years after oak failure. In 28 of 30 cases, female black bears that weighed less than 148 pounds on Oct. 1 produced no cubs, while those weighing more than 176 pounds successfully reproduced. This weight threshold represents a critical survival mechanism. Bears may physiologically assess their supply of stored nutrients in the fall and prevent their own egg implantation when stores are too low to support both themselves and their young through the denning period.
A single year of acorn failure creates ripple effects that can last for years through bear populations. The dramatic acorn failure of 1984 in the Southern Appalachians resulted in essentially 100% failure in cub production the following year. But the consequences didn’t end there. The low cub production in 1985 led to an unusually large recruitment in 1986, which in turn created an exceptionally large cohort of subadults dispersing in 1989. This demographic wave manifested as increased panhandling behavior in Great Smoky Mountains National Park, more bear incidents outside protected areas, and higher hunter kills during the fall hunting season. These patterns ultimately shape the long-term dynamics of North Carolina’s bear populations and our relationships to these animals, creating natural fluctuations that wildlife managers must account for in their conservation strategies.
As climate change alters precipitation patterns and temperature cycles across the Southeast, the traditional rhythms of mast production face new uncertainties. Earlier springs and more variable fall weather could disrupt the delicate timing that has evolved over millennia. For the bears of North Carolina, these small golden nuggets scattered across the forest floor represent not just a final feast before winter’s arrival, but a window into the intricate connections that bind forest ecosystems together.
While we humans prepare for autumn’s slower pace, the bears remind us that nature’s calendar operates on its own delicate timeline. One where survival depends on making the most of abundance when it arrives, acorn by precious acorn.
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